1. Field of the Invention
The present invention relates to a dot matrix printer which prints in a dot matrix form by striking a recording medium with dot-like striking members.
2. Description of the Prior Art
In order to print a character or the like with dots, n wires arranged vertically at a pitch P2 are shifted m times at a pitch P1 to form an m.times.n matrix, as shown in FIG. 1. The wires corresponding to the character or the like to be printed are driven upon each shift to print the character or the like.
FIGS. 2 to 5 show a conventional structure of a wire dot head for performing such dot printing.
Referring to FIGS. 2 to 5, a front guide 1 is fixed to an intermediate guide 2. The intermediate guide 2 has screw notches 2a for receiving screws for mounting the head on a carriage (not shown).
A flexible printed circuit board 3 formed integrally with a radiating fin plate 4 is arranged behind the intermediate guide 2.
The radiating fin plate 4 also serves as a spacer between the flexible printed circuit board 3 and an elliptical cylinder yoke 5. The radiating fin plate 4 transmits the heat of the yoke 5 to a back holder to be described later and comprises a material having a good thermal conductivity (e.g., an aluminum plate of about 2 mm thickness).
A plurality of cores 6 are fitted in the yoke 5. Bobbins 8 having coils 7 wound thereon are mounted on the respective cores 6.
Two terminals 8a are formed for each bobbin 8 and are soldered at one end thereof to the two ends of the coil 7. The other end of each terminal 8a is soldered to the flexible printed circuit board 3 through the yoke 5 and the radiating fin plate 4.
A rear guide 9 has an overall cylindrical shape. The rear guide 9 is fitted in the intermediate guide 2 though the yoke 5, the radiating fin plate 4, and the flexible printed circuit board 3.
One end of each return spring 10 is fixed to the rear end of the rear guide 9. The open end of the rear end portion of the yoke 5 is closed with an auxiliary yoke 11. A circular opening is formed at the central portion of the auxiliary yoke 11. The rear end of the rear guide 9 corresponds to the circular opening of the auxiliary yoke 11.
A post 8b projecting rearward from the bobbin 8 is fitted in a corresponding through hole formed in the auxiliary yoke 11 to project rearward. The rear end portions of armatures 14 are fitted around the posts 8b. The armatures 14 number the same as wires 22. A plunger 13 projects from an intermediate portion of each armature 14.
A spacer 12 is arranged between the auxiliary yoke 11 and the armatures 14.
The spacer 12 is incorporated so as to prevent wear by contact between the auxiliary yoke 11 and the armatures 14 and unstable movement of the armatures 14 due to residual magnetism or the like of the auxiliary yoke 11 upon formation of a closed magnetic circuit.
As shown in FIG. 5, the plurality of armatures 14 extend radially. The proximal end of each wire 22 is fixed to the distal end of the corresponding armature 14. The return spring 10 is elastically mounted between the free end of the armature 14 and the rear end of the rear guide 9. The return spring 10 normally biases the free end of the armature 14 in a direction to separate it from the rear end of the rear guide 9.
A stopper 15 is arranged behind the armatures 14 so as to oppose the free ends of the armatures 14. The stopper 15 is fitted in an armature holder 16. The distal end of each radial arm of the armature holder 16 presses the proximal end of the armature 14 toward the auxiliary yoke 11. The pressing force of each arm of the armature holer 16 against the armature 14 is given by a radial leaf spring 17 arranged behind the armature holder 16. A damper 18 is arranged behind the leaf spring 17. The damper 18 is made of a material having a large specific gravity such as lead and is in contact with a back holder 21 through a washer 19 and a spring washer 20.
In order to provide good heat dissipation for the head, the back holder 21 has a large surface area and has a number of fins on its surface, as shown in FIG. 3.
The back holder 21 is fixed to the intermediate guide 2 with screws 23 and covers a portion including the armature and the yoke.
The damper 18 is kept displaceable by means of the leaf spring 17 and the spring washer 20 so as to provide a vibration-free structure.
The washer 19 prevents forced pressing of the washer 20 into the damper 18.
In order to prevent buckling, the wires 22 are guided by the guides 1, 2 and 9 such that they gradually form a vertical array and terminate in a complete vertical array within the front guide 1.
Printing is performed with the head having the above structure. When the coil 7 is energized in accordance with a printing command, a closed magnetic circuit is formed by the yoke 5, the auxiliary yoke 11, the plungers 13, and the cores 6. Therefore, the plungers 13 are attracted toward the cores 6. The armatures 14 then allow the wires 22 to project a predetermined distance from the front guide 1 against the biasing force of the return springs 10. Then, the wires 22 dot-print through an ink ribbon (not shown) onto a printing paper sheet.
A magnetic circuit of a conventional wire dot head consists of the yoke 5, the core 6, the plunger 13, and the auxiliary yoke 11, as shown in FIG. 6.
With this structure, when only the drive portion of the armatures 14 is considered, the printing force and the response characteristics to an electrical signal depend only upon a gap G between the core 6 and the plunger 13. The gap G is given by: EQU G=H.sub.1 +t.sub.1 +t.sub.2 -H.sub.2 -H.sub.3
where H.sub.1 is the distance between the bottom surface and the end face of the yoke 5, H.sub.2 is the height of the core 6, H.sub.3 is the height of the plunger 13, t.sub.1 is the thickness of the auxiliary yoke 11, and t.sub.2 is the thickness of the spacer 12 comprising a polyester film.
As can be seen from FIG. 2, the auxiliary yoke 11 serves as a stopper for the armatures 14 along the printing direction. As is well known, the printing force of the wires which allows copying of 3 to 4 sheets is about 2.5 kg per wire.
Therefore, when all of the plurality of wires 22 are driven at the same time, an extermely large force acts on the side surfaces of the auxiliary yoke 11.
The displacement of the side surface of the auxiliary yoke 11 will be calculated with a model in accordance with the force acting on the side of the auxiliary yoke 11 of the conventional printing head.
As shown in FIG. 7, when the attracting force of each armature 14 is represented by F and the number of wires (the number of armatures) is represented by N, the force F.sub.T acting on the auxiliary yoke 11 due to the N armatures is given by: EQU F.sub.T =NF (1)
For the sake of simplicity, the outer and inner diameters of the auxiliary yoke 11 are represented by D.sub.0 and D.sub.1, and it is assumed that the force F.sub.T acts as an equally distributed weight w on the side surface of the auxiliary yoke 11, as shown in FIG. 8.
As can be seen from FIG. 2, it is assumed that the periphery of the auxiliary yoke 11 is simply supported.
Under these assumptions, the equally distributed weight w (kg/mm.sup.2) is given by: EQU w=F.sub.T /((.pi./4)(D.sub.0.sup.2 -D.sub.1.sup.2)) (2)
A displacement t at the inner periphery of the auxiliary yoke 11 is given by: EQU t=.alpha..sub.13 xwD.sub.0.sup.4 /Eh.sup.3 =0.78.times.(wD.sub.0.sup.4 /Eh.sup.3) (3)
where E is the Young's modulus of the material of the auxiliary yoke, h is the thickness of the auxiliary yoke, and .alpha..sub.13 is the flexural coefficient. When the ratio D.sub.1 /D.sub.0 is 0.42, .alpha..sub.13 is 0.78.
When the distance between the plunger position on the auxiliary yoke 11 to the outer periphery thereof is represented by l, a displacement t.sub.p at the plunger position is given by: EQU t.sub.p 2tl/(D.sub.0 -D.sub.1) (4)
When the auxiliary yoke 11 is displaced, a gap G.sub.2 between the core 6 and the plunger 13 is given by: EQU G.sub.2 =G.sub.1 t.sub.p ( 5)
where G.sub.1 is the gap to when the auxiliary yoke 11 is not displaced.
As can be seen from equation (5) above, G.sub.2 can be zero. This corresponds to a case wherein the core and the plunger are in contact with each other.
In such a case, even if the coil is deenergized, the core 6 and the plunger 13 attract each other due to residual magnetism. Therefore, the wire 22 cannot easily project from the front guide. Then, the wire 22 it tightly urged against the ink ribbon, thus resulting in damage to the ink ribbon or the wire 22 itself.
When the above-mentioned structure is adopted, the posts 8b of the bobbins 8 prevent an improvement in magnetic efficiency.
The magnetic circuit for driving the wires 22 comprises the cores 6, the yoke 5, the auxiliary yoke 11, and the plungers 13.
Holes 11a for receiving the posts 8b are formed in the auxiliary yoke 11. The armatures 14 pivot about the posts 8b inserted in the holes 11a.
The posts 8b are formed integrally with the bobbins 8 and are made of a nonmagnetic insulating resin or the like. When the armatures 14 pivot about the posts 8b, the posts 8b wear at the edges of the holes of the armatures 14 made of iron plates. In the worst case, the posts 8b of the bobbins 8 may be so damaged as to prevent further printing.
Furthermore, since the holes 11a are formed at a position intermediate in the magnetic path, the magnetic flux is not formed at this portion, as shown in FIG. 9.
Furthermore, due to the presence of the holes 11a, the reluctance is increased, the magnetic efficiency is lowered, and the printing duty cannot be improved above a predetermined level.
In a conventional head having the structure described above, the shape and the arrangement of the core 6 and the plunger 13 prevent a higher density of the wires, a compact wire assembly, and a higher efficiency of the printer.
As shown in FIGS. 4 and 5, the cores 6 each having a circular section are arranged on an elliptical track in the elliptical cylinder yoke 5.
The bobbins 8 are fitted around the cores 6, and the plungers 13 integral with the armatures 14 are fitted inside the bobbins 8.
The plunger 13 also has a circular section as shown enlarged in FIG. 10. Therefore, the plunger 13 together with the core 6 requires a space corresponding to its diameter. For this reason, it is difficult to integrate at high density the cores 6 and the plungers 13, and a compact wire dot head having a large number of pins and an excellent power-printing force converting efficiency cannot be obtained.
Moreover, In the wire dot head having the above-mentioned structure, the presence of the auxiliary yoke 11 results in a high noise. The auxiliary yoke 11 serves as a stopper in the printing direction when the armatures 14 perform the printing operation.
However, since the auxiliary yoke 11 comprises a metal, the auxiliary yoke 11 generates a high noise upon contacting with the armatures 14.
Although the polyester film spacer 12 is interposed between the auxiliary yoke 11 and the armatures 14, it does not provide any substantial noise preventing effect due to its small thickness of 50 .mu.m.
In the normal printing state, since the entire stroke of the wires 22 is not utilized, the armatures 14 do not contact the auxiliary yoke 11. However, if the platen has a low hardness, the wires 22 are strongly urged against the platen, or if the wires 22 buckle, the auxiliary yoke 11 will contact the armatures 14.
When printing of only one sheet is to be performed with the gap between the platen and the distal end of the head adjusted for printing a plurality of sheets, the armatures 14 contact the auxiliary yoke 11 with a strong force, thus generating a large impact noise.